The invention relates to a bootstrap reference circuit and, in particular, to a bootstrap reference circuit using a shunt regulator biased by a peaking current source for achieving high supply rejection ratio and zero temperature coefficient.
Electronic circuits often require a voltage reference that is stable and substantially constant over temperature and power supply variations. A bandgap reference circuit is typically used to generate such a temperature-independent and power-supply-independent reference voltage. A bandgap reference circuit generates a bandgap voltage of 1.24 volts by developing a first voltage related to a multiple of the base-to-emitter voltage differential (xcex94VBE) of a pair of transistors operating at different current densities and a second voltage related to the base-to-emitter voltage VBEof a third transistor. The first voltage xcex94VBE is proportional to absolute temperature (PTAT) and thus has a positive temperature coefficient. On the other hand, the second voltage VBE has a negative temperature coefficient. Thus, the sum of Kxcex94VBE (where K is a multiple) and the base-to-emitter voltage VBE produces a voltage that has nearly no temperature dependence and no power-supply dependence. An example of a bandgap voltage reference circuit is described in U.S. Pat. No. 4,447,784, which patent is incorporated herein by reference in its entirety.
In electronic circuits including high gain circuit components, it is important for the reference voltage to have a high power supply rejection ratio (PSRR). One method of providing a reference voltage with high PSRR is to use a bandgap reference circuit as a shunt regulator. The bandgap voltage, at 1.24 volts, is bootstrapped to the desired voltage for powering the designated circuits. The most common method to supply current to such a bandgap, reference shunt regulator is to use a PTAT/R current. The PTAT/R current is derived from applying a PTAT voltage, such as the xcex94VBE voltage of the bandgap reference circuit, to a resistor R.
The conventional method of providing a high PSRR voltage reference has several shortcomings. First, because the bootstrap current (that is, the PTAT/R current) is xe2x80x9cbounced offxe2x80x9d the power supply voltage through the resistor R, the bootstrap current increases as the bandgap voltage increases. As a result, as the bandgap reference circuit is powering up, the bandgap reference circuit is destabilized because of positive feedback from the bootstrap current. Even with this positive feedback, the conventional bandgap reference circuit will still be able to regulate because the gain of the amplifier in the bandgap reference circuit is typically capable of overcoming the gain of the bootstrap current. However, increased compensation capacitance has to be added to stabilize the bandgap reference circuit which has the effect of slowing down the response of the bandgap reference circuit.
Therefore, a reference circuit capable of achieving high PSRR without the aforementioned disadvantages is desired.
According to one embodiment of the present invention, a circuit includes a shunt regulator for generating a reference voltage at a first node, a current source generating a current, and a current mirror coupling the current to the shunt regulator for supplying the shunt regulator. In operation, when the shunt regulator is powering up, the current has an increasing magnitude when the voltage at the first node is less than a predefined voltage value where the predefined voltage value is less than the reference voltage. Furthermore, the current has a decreasing magnitude when the voltage at the first node is greater than the predefined voltage value.
In one embodiment, the shunt regulator includes a bandgap reference circuit and the reference voltage is a bandgap voltage. In this case, the predefined voltage value can be set to 1 volt.
According to one embodiment of the present invention, the current source includes a first resistor coupled between the first node and a second node, a second resistor coupled between the second node and a third node, a first transistor having a first current handling terminal coupled to the third node, a second current handling terminal coupled to a first supply voltage, and a control terminal coupled to the second node, and a second transistor having a first current handling terminal coupled to generate the current, a second current handling terminal coupled to the first supply voltage, and a control terminal coupled to the third node.
According to one aspect of the present invention, the predefined voltage value at which the current generated by the current source has a peak value is established by the resistance of the first and second resistors. Specifically, when the current has a peak current value equaling to VT/R2, where VT is the thermal voltage (kT/q) and R2 is the resistance of the second resistor, the predefined voltage value is the sum of a voltage at the second node and a voltage across the first resistor at the peak current value.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.